Recently, luchonacho asked about the phrase in saecula saeculōrum: literally, "into the lifetimes of the lifetimes". It's kind of a weird phrase, for multiple reasons.
However, I'd always thought the phrase was in saeculō saeculōrum: using singular instead of plural, and ablative instead of accusative. With singular, it's a common Hebraism (as in "the holy of holies", "the king of kings", "the song of songs"), and with the ablative, it means "in" (as in, "during") instead of "into".
It seems I may have fallen prey to the Mandela Effect, as I can't find that particular phrasing anywhere. Did I just make it up? Or does the version with ablative singular actually exist somewhere?